An image display such as a liquid crystal display employs a polarizer (polarizing plate) according to the underlying principle of the displays. In company with progress to a large-sized image display or a diversification of functionality thereof in recent years, the demand for polarizers have been on the rise and at the same time, requirements for better quality and improved durability have been more and more increasing. Especially, very high heat resistance has been required in applications to the liquid crystal displays, which are assumed to be used in a severe environment outdoors, used in a cellular phone, a PDA and the like; and to the liquid crystal displays used in a vehicle navigation and a liquid crystal projector.
Conventionally, as a polarizer for an image display, there has been widely employed mainly a polarizer fabricated by dyeing a stretched polyvinyl alcohol film with a dichroic material such as iodine or a dye, which has the property of dichroism (see, for example, JP-A No. 2001-296427).
An iodine-based polarizer is obtained in a procedure in which a film is dyed with an aqueous solution containing amorphous iodine, followed by stretching, has a high polarizability for visible light and can be fabricated as a large-sized polarizer. However, in the iodine-based polarizer iodine sublimes at a high temperature or its complex structure is altered, which makes it difficult to maintain a polarization performance. On the other hand, a dye-based polarizer using a dichroic dye is better in heat resistance as compared with the iodine-based polarizer, whereas the polarizer has not been widely employed with limited applications thereof because of insufficient dichroic ratio of a dye used therein and poor weather resistance thereof. Note that in addition to polyvinyl alcohols, examples of film materials of a polarizer include: polystyrenes, cellulose derivatives, polyvinyl chlorides, polypropylenes, acrylic-based polymers, polyamides, polyesters, saponified products of ethylene-vinyl acetate copolymers and others.
A polarizer fabricated by dispersing metallic particles having anisotropy in light absorption property on an isotropic substrate is adopted as a polarizer used in the field of optical devices such as an optical communication device and an optical recording reproduction device requiring a heat resistance at a high temperature. As such a polarizer, there has been adopted, for example, a polarizer fabricated in a procedure in which metal particles are deposited in glass through a reduction reaction or the like, followed by stretching. A polarizer obtained by dispersing fine metallic particles on an isotropic substrate is not suited for mass production because fine metallic particles are deposited with a vacuum deposition method or the like, which requires a high temperature heating process.
It has been known that a film in which fine metallic particles having anisotropy are dispersed in polyimide are uniaxially stretched to thereby obtain a polarization film good in heat resistance (see, for example, JP-A No. 8-184701). Since such a polarizing film is, however, made form polyimide, the film has a problem of yellowing even after uniaxially stretching and poor in transmittivity.
A polarizer using a dichroic material described above to which iodine belongs exerts a polarization performance by aligning the dichroic material along a stretch direction. Absorption spectra measured when polarized light enters on such a polarizer generally include an absorption spectrum (MD spectrum) with an incident polarization plane in parallel to a stretch direction of the polarizer and an absorption spectrum (TD spectrum) with an incident polarization plane in parallel to a direction perpendicular to the stretch direction of the polarizer, wherein both have the same spectral line shape as each other (almost the same absorption peak wavelength) and a relation in absorbance of MD spectrum>TD spectrum. That is, “the absorption peak of an absorption spectrum shifts longitudinally” depending on an azimuth of the incident polarization plane relative to a polarizer. In order to enhance a polarization performance, it is required that an absorbance at the absorption peak in the MD spectrum is increased, while an absorbance at the absorption peak in the TD spectrum is decreased to the lowest possible value. That is, it has been necessary to increase a difference in absorbance between the MD spectrum and the TD spectrum to the maximum possible value.